Within the field of electronic circuits there is often a need for a current boost to either a current source or a current sink. For example, circuits that require a low supply voltage may employ an output stage designed to work well with low supply voltages, but that may suffer from limited output current.
FIG. 1 depicts a typical emitter follower (or common collector) output stage, which may not be suitable for low voltage applications. The output voltage swing is limited by the base to emitter voltage drop of each output transistor. Thus, the voltage swing is limited by about 1.4 volts. For applications that can use a relatively large voltage source, the Vbe voltage drops do not pose a problem. However, some applications require a lower voltage source. For example, battery powered application may only have about 1.8 volts between the positive and negative voltage supply. This would leave a maximum voltage swing of only 0.4 volts, which may be unacceptably low. Furthermore, other circuit elements (not depicted in FIG. 1) may also impact the possible voltage swing.
To achieve a higher output voltage swing, a common emitter output stage may be used to achieve what is referred to as a rail-to-rail output. In a similar fashion, a common source configuration may be used for field effect transistors. Referring to FIG. 2, the voltage drop from the collector to the emitter can be on the order of tens of millivolts when the transistor is in saturation (Vce(sat)). This allows the output node to swing between almost the entire range of the positive and negative voltage supply nodes, hence the term rail-to-rail configuration.
The common emitter (or common source) configuration provides nearly the widest voltage swing possible in terms of the voltage supply, and is thus desirable for low voltage supply applications. However, even with this configuration, the very low supply voltage may limit the output current sourced and/or sunk. For example, when the load resistance “Rload” is small, the circuit must source (or sink) a large current. However, this may require a larger base-to-emitter voltage (Vbe) than is possible given the low supply voltage. To illustrate this, for every 60 mV change in Vbe, the collector current, and hence output current, may change by a factor of about ten. Depending on device fabrication, when Vbe is about 0.7 V, the output current may be on the order of 10 micro-amperes. For the collector current to reach 10 milli-amperes, Vbe may need to be about 0.86 V. However, even this relatively small change in Vbe may not be possible given a low supply voltage along with various elements other than the output transistors (not shown in FIG. 2) having voltage drops.
Thus, there is a need to provide a current boost to a circuit. While a current boost may be valuable to a common-emitter output stage, those of ordinary skill in the art will appreciate that this need is not limited to such configurations. For example, the circuit of FIG. 1 may benefit from a current boost in some applications. Also circuits employing field effect devices may benefit from a current boost.